The word nail has two main meanings:

1. A thin piece of metal with a flat head on one end and a sharp point on the other, used to fasten things together. When you hammer a nail into wood, you're joining two pieces or hanging something on a wall. Nails have been used in construction for thousands of years. Ancient Roman nails look remarkably similar to modern ones because the design works so well. Carpenters might use tiny finishing nails for delicate work or large framing nails to build the skeleton of a house.
2. The hard covering that protects the end of your fingers and toes. Your fingernails and toenails are made of keratin, the same material as animal claws and hooves. They grow continuously throughout your life, which is why you need to trim them regularly.

The word also appears in several expressions. When you nail something, you do it perfectly: “She really nailed that piano performance.” When someone hits the nail on the head, they've said exactly the right thing. And if you're extremely nervous, you might bite your nails, a habit that means you're chewing on your fingernails without thinking about it.